Thin panel speakers can be used to emit sound due to propagation of bending waves in the panel by one or more transducers. Such distributed mode loudspeakers (DMLs) may have various advantages over conventional speakers, such as the ability to create a high sound pressure level (SPL) with a relatively small local displacement of the panel due to increased panel surface area. The transducers, or exciters, used to generate the bending waves can thus be small, and the entire speaker device can be thin, such that the panel speaker can be used in various emerging technologies, for example, speaking picture frames.
Similar to other panel type loudspeakers, such as electrostatic or planar magnetic speakers, DMLs can create a deep soundstage and a large “sweet spot” (e.g., area in the listening room with the best sound). However, the performance of DMLs in terms of sound quality can suffer from a number of shortcomings, such as non-flat frequency, randomized phase response, fast changes in frequency response with direction (listening position), and/or long sound decay times. These shortcomings may originate from one or more fundamental design features. For example, because it takes a finite amount of time for a bending wave in the panel to propagate from the excitation point (exciter/transducer attachment location) to the edge of the panel, sharp transients in the sound waveform can stretch in time. This stretch can also be frequency-dependent, since bending waves can be highly dispersive due to quickly travelling higher frequencies. Moreover, reflections from the edges of the panel can lead to the formation of distinct resonances in the frequency response, which can correspond to the mechanical modes of the panel. Finally, if the damping or wave attenuation in the panel material is not sufficiently high, the edge reflections can also cause “ringing” (the persistence of standing waves and the sound they emit after the excitation has stopped).
DMLs using only travelling bending waves have been developed in an attempt to remedy various drawbacks discussed above. For example, by introducing incisions precisely cut at the edges of a rectangular panel, or by using star-shaped diffusers at the outer edge of a circular panel, edge reflections may be passively suppressed. These designs can provide a flatter frequency and/or smoother phase response and/or can limit the distortion of sharp transients, such as ringing and/or modal resonances. However, these approaches can be difficult to implement in the case of a thin glass panel and/or a rectangular panel (e.g., a bar having short and long sides). Cutting precise slots in glass can be complex, time-consuming, and can compromise reliability of the panel. Moreover, in the case of a small rectangular panel, there may not be enough space along the short side for properly designed star-shaped or saw-tooth shaped diffusers. Further, incisions and/or star-shaped diffusers can diminish the aesthetic appearance of a glass panel, e.g., a transparent glass panel.
Accordingly, it would be advantageous to provide an exemplary glass loudspeaker which does not suffer from the disadvantages associated with conventional DMLs. It would also be advantageous to provide glass speakers which can utilize the advantage of non-resonant design while still preserving aesthetic display qualities.